Half-power buffer amplifier

ABSTRACT

A half-power buffer amplifier includes a buffer stage having a first-half buffer stage and a second-half buffer stage. An output of the first-half buffer stage is controllably fed back to a rail-to-rail differential amplifier, and an output of the second-half buffer stage is controllably fed back to the rail-to-rail differential amplifier. A switch network controls the connection between the outputs of the buffer stage and an output node of the half-power buffer amplifier in a manner such that a same pixel, with respect to different frames, of a display panel is driven by the same rail-to-rail differential amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a buffer amplifier, and moreparticularly, to a half-power buffer amplifier adaptable to a liquidcrystal display (LCD).

2. Description of Related Art

A liquid crystal display (LCD) typically includes rows and columns ofpicture elements (or pixels) arranged in matrix form. Each pixelincludes a thin film transistor (TFT) and a pixel electrode formed on asubstrate (or panel). The gates of the TFTs in the same row areconnected together through a gate line, and controlled by a gate driver(or scan driver). The sources of the TFTs in the same column areconnected together through a source line, and controlled by a sourcedriver (or data driver). A common electrode is formed on anothersubstrate (or panel). A liquid crystal (LC) layer is sealed between thepixel electrode substrate and the common electrode substrate, and foreach pixel the voltage difference between the pixel electrode and thecommon electrode determines the display of the pixel(s).

As the resolution of the LCD increases, thousands of output bufferamplifiers or buffer circuits should be built into the source driver. Asa result, the LCD, particularly the large-size and/or high-resolutionLCD, consumes immense power. On the other hand, as the power is preciousto any portable electronic device with such an LCD, the powerconsumption of the LCD therefore tends to determine the available runtime of the entire portable electronic device. Accordingly, an LCD withlow-power buffer amplifiers is becoming indispensable, and some schemes,such as half-power buffer amplifiers have been proposed.

FIGS. 1A-1B show conventional half-power buffer amplifiers 10 and 12.Regarding the display of a first frame, referring to FIG. 1A, theamplifier 10 for the first channel CH1 generates the first half power(e.g., VDD to VDD/2) as the output OUT1 through a switch S1 (asindicated by the solid arrow). At the same time, the amplifier 12 forthe second channel CH2 generates the second half power (e.g., VDD/2 toground) as the output OUT2 through a switch S4 (as indicated by thesolid arrow).

Subsequently, regarding the display of a second frame, the amplifier 12for the second channel CH2 generates the second half power (e.g., VDD/2to ground) as the output OUT1 through a switch S3 (as indicated by thedashed arrow). At the same time, the amplifier 10 for the first channelCH1 generates the first half power (e.g., VDD to VDD/2) as the outputOUT2 through a switch S2 (as indicated by the dashed arrow).

As shown in FIG. 2, during the display of the first frame, the outputOUT1 is used to drive the first row of the LCD, and the output OUT2 isused to drive the second row of the LCD. Subsequently, during thedisplay of the second frame, the output OUT2 is used to drive the firstrow of the LCD, and the output OUT1 is used to drive the second row ofthe LCD. In other words, the same pixel (such as the circled pixel shownin FIG. 2) with respect to different frame(s) is driven by differentamplifier(s) 10/12. As a result, the offsets incurred from differentamplifiers cannot be properly cancelled, thereby degrading the displayquality.

For the reason that the conventional half-power amplifiers cannoteffectively cancel their offset voltages while decreasing powerconsumption, a need has thus arisen to propose a novel scheme or circuitin order to resolve the offset cancellation issue.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the embodiment of thepresent invention to provide a half-power amplifier that is capable ofeffectively improving offset cancellation while decreasing powerconsumption.

According to one embodiment, the half-power buffer amplifier includes arail-to-rail differential amplifier, a buffer stage, and a switchnetwork. The buffer stage includes a first-half buffer stage and asecond-half buffer stage, wherein an output of the first-half bufferstage is controllably fed back to the rail-to-rail differentialamplifier, and an output of the second-half buffer stage is controllablyfed back to the rail-to-rail differential amplifier. The switch networkcontrols the connection between the outputs of the buffer stage and anoutput node of the half-power buffer amplifier in a manner such that asame pixel, with respect to different frames, of a display panel isdriven by the same rail-to-rail differential amplifier. In one exemplaryembodiment, one buffer stage is associated with one rail-to-raildifferential amplifier for each channel. In another exemplaryembodiment, one buffer stage is associated with (or shared between) tworail-to-rail differential amplifiers of adjacent channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show conventional half-power buffer amplifiers;

FIG. 2 shows exemplary pixels driven by the buffer amplifiers of FIGS.1A-1B;

FIG. 3 shows a half-power buffer amplifier according to one embodimentof the present invention;

FIG. 4 shows exemplary pixels driven by the buffer amplifiers of FIG. 3;

FIG. 5 shows the first switch or the second switch implemented by atransmission gate (TG);

FIG. 6 shows a detailed circuit that exemplifies the half-power bufferamplifier according to the embodiment of the present invention; and

FIGS. 7A-7B show a half-power buffer amplifier according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a half-power buffer amplifier according to one embodimentof the present invention. Although the embodiment illustrated here isadapted to a source driver of a liquid crystal display (LCD) (nowshown), it is, however, appreciated by those skilled in the pertinentart that this embodiment (and other embodiments) may be well adapted toother display panels.

In the embodiment, at least one half-power buffer amplifier (abbreviatedas buffer amplifier hereinafter) is used in the source driver of theLCD. In the example, two buffer amplifiers, that is, a first bufferamplifier 3A and a second buffer amplifier 3B, are illustrated in thefigure for two channels CH1 and CH2 respectively. As shown in FIG. 4,the output OUT1 of the first buffer amplifier 3A is utilized as anoutput stage of the source driver to drive the first row of the LCD, andthe output OUT2 of the second buffer amplifier 3B is utilized as anotheroutput stage of the source driver to drive the second row of the LCD.

In the embodiment, the first buffer amplifier 3A includes a rail-to-raildifferential amplifier (abbreviated as differential amplifierhereinafter) 30A, a buffer stage 32A and a switch network 34A.Specifically, the differential amplifier 30A includes a rail-to-railoperational amplifier (OP-Amp). In this specification, the term “rail”means the highest level (e.g., VDD) or the lowest level (e.g., ground)of a power supply. Accordingly, the rail-to-rail differential amplifier30A is an OP-Amp that is powered by a full-range (e.g., VDD to ground)power supply, and the input voltage level is within the full range ofthe power supply. In another example, the full range of the power supplymay be from VDD to VSS.

The buffer stage 32A includes a first-half buffer stage 320A and asecond-half buffer stage 322A. With respect to the first-half bufferstage 320A, it is coupled to the power (e.g., VDD) at one end andcoupled to half power plus a predetermined (non-zero) voltage ΔV (e.g.,VDD/2+ΔV) at the other end. The first-half buffer stage 320A generatesan output OUTA that is controllably coupled to the inverting input nodeof the differential amplifier 30A while the non-inverting input node ofthe differential amplifier 30A is configured to receive an input IN1. Inthe embodiment, the first-half buffer stage 320A includesserial-connected P-type transistor P320A and N-type transistor N320A.Specifically, the first source/drain of the P-type transistor P320A iscoupled to the power, the second source/drain of the P-type transistorP320A is coupled to the first source/drain of the N-type transistorN320A, and the second source/drain of the N-type transistor N320A iscoupled to the half power plus ΔV. The gate of the P-type transistorP320A and the gate of the N-type transistor N320A are under control ofthe differential amplifier 30A.

Similarly, with respect to the second-half buffer stage 322A, it iscoupled to the half power minus a predetermined voltage ΔV (e.g.,VDD/2-ΔV) at one end and connected to the ground at the other end. Thesecond-half buffer stage 322A generates an output OUTB that iscontrollably coupled to the inverting input node of the differentialamplifier 30A. In the embodiment, the second-half buffer stage 322Aincludes serially-connected P-type transistor P322A and N-typetransistor N322A. Specifically, the first source/drain of the P-typetransistor P322A is coupled to the half power minus ΔV, the secondsource/drain of the P-type transistor P322A is coupled to the firstsource/drain of the N-type transistor N322A, and the second source/drainof the N-type transistor N322A is coupled to the ground. The gate of theP-type transistor P322A and the gate of the N-type transistor N322A areunder control of the differential amplifier 30A.

The switch network 34A includes a first switch 340A and a second switch342A. The first switch 340A controls the connection between the outputOUTA of the first-half buffer stage 320A and the output OUT1 of thefirst buffer amplifier 3A; and the second switch 342A controls theconnection between the output OUTB of the second-half buffer stage 322Aand the output OUT1 of the first buffer amplifier 3A. The first switch340A or the second switch 342A may be implemented by, but not limitedto, a transmission gate (TG) as shown in FIG. 5.

In the same manner, the second buffer amplifier 3B for the secondchannel CH2 has the same composing elements and configurations as thefirst buffer amplifier 3A for the first channel CH1, and, therefore,same numerals are adapted except that “IN1” is replaced with “IN2,”“OUTA/B” is replaced with “OUTC/D,” “OUT1” is replaced with “OUT2” andall As are replaced with Bs. The description of the composing elementsand their configurations are omitted for brevity.

FIG. 6 shows a detailed circuit that exemplifies the first bufferamplifier 3A according to the embodiment of the present invention. Inthe embodiment, the rail-to-rail differential amplifier 30A includes apair of complementary differential amplifiers 300A and an amplifyingstage 302A. Specifically, the complementary differential amplifiers 300Areceive the input IN1 at the non-inverting input node (+), and receivethe output OUTA/OUTB of the first/second-half buffer stage 320A/322A atthe inverting input node (−). The amplifying stage 302A is used toamplify the voltage difference of the inputs (e.g., IN1 and OUTA/OUTB),and control the first/second-half buffer stage 320A/322A. According toone aspect of the present embodiment, the amplifying stage 302A includesa pair of floating current sources (or level shifters), that is, a firstfloating current source 3020A and a second floating current source3022A, which are used to control the first-half buffer stage 320A andthe second-half buffer stage 322A respectively. Specifically, in theembodiment, the first floating current source 3020A includes aparallel-connected N-type transistor N1 and P-type transistor P1 withtheir sources/drains respectively connected at nodes M1 and N1. The nodeM1 is coupled to the gate of the P-type transistor P320A of thefirst-half buffer stage 320A, and the node N1 is coupled to the gate ofthe N-type transistor N320A of the first-half buffer stage 320A.Generally speaking, the floating current source 3020A is an AB classoperated circuit, in which the P-type transistor shuts off and theN-type transistor turns on when a positive current (i.e., a currentflowing downward in the figure) occurs; on the other hand, the P-typetransistor turns on and the N-type transistor shuts off when a negativecurrent (i.e., a current flowing upward in the figure) occurs.

Similarly, the second floating current source 3022A includes aparallel-connected N-type transistor N2 and P-type transistor P2 withtheir sources/drains respectively connected at nodes M2 and N2. The nodeM2 is coupled to the gate of the P-type transistor P322A of thesecond-half buffer stage 322A, and the node N2 is coupled to the gate ofthe N-type transistor N322A of the second-half buffer stage 322A. In thesame manner, the second buffer amplifier 3B for the second channel CH2may have the same compositional elements and configurations as the firstbuffer amplifier 3A for the first channel CH1, and, therefore,description of the compositional elements and their configurations isomitted for brevity.

In the operation of the first/second buffer amplifiers 3A and 3B for thedisplay of a first frame, referring to the first buffer amplifier 3A forthe first channel CH1 in FIG. 3, the first switch 340A turns on and thesecond switch 342A shuts off, and, accordingly, the output OUT1 of thefirst buffer amplifiers 3A drives the first row of the LCD (FIG. 4)through the first switch 340A (as indicated by the solid arrow), whilethe path along the second switch 342A is blocked. More specifically,referring to FIG. 6, the inverting input node (−) of the complementarydifferential amplifiers 300A receives the output OUTA of the first-halfbuffer stage 320A, a P-type transistor P3 and an N-type transistor N3(in the amplifying stage 302A) are properly biased, while a P-typetransistor P4 (in the amplifying stage 302A) shuts off by connecting itsgate to the power and an N-type transistor N4 shuts off by connectingits gate to the ground. In other words, the first/second floatingcurrent source 3020A/3022A shuts off and accordingly shuts off theassociated buffer stage 320A/322A when the associated switch 340A/342Ashuts off.

At the same time, referring to the second buffer amplifier 3B for thesecond channel CH2 in FIG. 3, the first switch 340B shuts off and thesecond switch 342B turns on, and, accordingly, the output OUT2 of thesecond buffer amplifiers 3B drives the second row of the LCD (FIG. 4)through the second switch 342B (as indicated by the solid arrow), whilethe path along the first switch 340B is blocked.

Subsequently, for the display of a second frame, referring to the firstbuffer amplifier 3A for the first channel CH1 in FIG. 3, the firstswitch 340A shuts off and the second switch 342A turns on, and,accordingly, the output OUT1 of the first buffer amplifiers 3A drivesthe first row of the LCD (FIG. 4) through the second switch 342A (asindicated by the dashed arrow), while the path along the first switch340A is blocked. More specifically, referring to FIG. 6, the invertinginput node (−) of the complementary differential amplifiers 300Areceives the output OUTB of the second-half buffer stage 322A, a P-typetransistor P4 and an N-type transistor N4 (in the amplifying stage 302A)are properly biased, while a P-type transistor P3 (in the amplifyingstage 302A) shuts off by connecting its gate to the power and an N-typetransistor N3 shuts off by connecting its gate to the ground.

At the same time, referring to the second buffer amplifier 3B for thesecond channel CH2 in FIG. 3, the first switch 340B turns on and thesecond switch 342B shuts off, and, accordingly, the output OUT2 of thesecond buffer amplifiers 3B drives the second row of the LCD (FIG. 4)through the first switch 340B (as indicated by the dashed arrow), whilethe path along the second switch 342B is blocked.

According to the embodiment described above, as the first-half bufferstage 320A/320B provides approximately half range (i.e., from the powerto the half power+ΔV) of the entire power supply and the second-halfbuffer stage 322A/322B provides approximately another half range (i.e.,from the half power-AV to the ground) of the entire power supply, theoverall power consumption may thus be substantially decreased andtemperature of the circuit may be accordingly lowered. Furthermore, assame pixel (such as the circled pixel shown in FIG. 4) of differentframe may be driven by the same differential amplifier 30A/30B, theoffset cancellation of the pixel may therefore be substantiallyimproved.

FIGS. 7A-7B show a half-power buffer amplifier according to anotherembodiment of the present invention. The present embodiment is similarto the previous embodiment except that a single buffer stage 32 may beshared between adjacent channels. Specifically, referring to FIG. 7A,with respect to the display of a first frame, the first-half bufferstage 320 is controlled by the differential amplifier 30A for the firstchannel CH1, in order to provide power, through the first switch 340A ofthe switch network 34A, to generate the output OUT1 (as indicated by thesolid arrow). At the same time, the second-half buffer stage 322 iscontrolled by the differential amplifier 30B for the second channel CH2,in order to provide power, through the second switch 342B of the switchnetwork 34B, to generate the output OUT2 (as indicated by the solidarrow).

Referring to FIG. 7B, with respect to the display of a second frame, thefirst-half buffer stage 320 is controlled by the differential amplifier30B for the second channel CH2, in order to provide power, through thesecond switch 342A of the switch network 34B, to generate the outputOUT1 (as indicated by the dashed arrow). At the same time, thesecond-half buffer stage 322 is controlled by the differential amplifier30A for the first channel CH1, in order to provide power, through thefirst switch 340B of the switch network 34B, to generate the output OUT2(as indicated by the dashed arrow).

The first buffer amplifier 30A or the second buffer amplifier 30B inFIG. 7A-7B may be implemented with the complementary differentialamplifier 300A and the amplifying stage 302A as shown in FIG. 6.Particularly, each amplifying stage (e.g., 302A) includes, among others,a first floating current source (e.g., 3020A) and a second floatingcurrent source (e.g., 3022A). In operation, the first floating currentsource of the first buffer amplifier 30A turns on to control thefirst-half buffer stage 320 (FIG. 7A), while the second floating currentsource of the first buffer amplifier 30A shuts off. At the same time,the second floating current source of the second buffer amplifier 30Bturns on to control the second-half buffer stage 322 (FIG. 7A), whilethe first floating current source of the second buffer amplifier 30Bshuts off.

Subsequently, the second floating current source of the first bufferamplifier 30A turns on to control the second-half buffer stage 322 (FIG.7B), while the first floating current source of the first bufferamplifier 30A shuts off. At the same time, the first floating currentsource of the second buffer amplifier 30B turns on to control thefirst-half buffer stage 320 (FIG. 7B), while the second floating currentsource of the second buffer amplifier 30B shuts off.

According to the present embodiment, the advantages (e.g., decreasedpower consumption, lowered temperature and improved offset cancellation)of the previous embodiment may be maintained, and, furthermore, chiparea may be further decreased due to the sharing of the buffer stagebetween adjacent channels.

Although specific embodiments have been illustrated and described, itwill be appreciated by those skilled in the art that variousmodifications may be made without departing from the scope of thepresent invention, which is intended to be limited solely by theappended claims.

1. A half-power buffer amplifier, comprising: a rail-to-raildifferential amplifier; a buffer stage that includes a first-half bufferstage and a second-half buffer stage, wherein an output of thefirst-half buffer stage is controllably fed back to the rail-to-raildifferential amplifier, and an output of the second-half buffer stage iscontrollably fed back to the rail-to-rail differential amplifier; and aswitch network configured to control connection between the outputs ofthe buffer stage and an output node of the half-power buffer amplifierin a manner such that a same pixel, with respect to different frames, ofa display panel is driven by the same rail-to-rail differentialamplifier.
 2. The half-power buffer amplifier of claim 1, wherein therail-to-rail differential amplifier comprises an operational amplifier.3. The half-power buffer amplifier of claim 2, wherein the operationalamplifier is powered by a full-range power supply, and input voltagelevel of the operational amplifier is within the full range of the powersupply.
 4. The half-power buffer amplifier of claim 2, wherein theoutput of the first-half buffer stage or the output of the second-halfbuffer stage is controllably coupled to an inverting input node of theoperational amplifier.
 5. The half-power buffer amplifier of claim 1,wherein the first-half buffer stage is coupled to power at one end andcoupled to half power plus a predetermined voltage (ΔV) at another end,and the second-half buffer stage is coupled to the half power minus thepredetermined voltage (ΔV) at one end and coupled to ground at anotherend.
 6. The half-power buffer amplifier of claim 5, wherein thefirst-half buffer stage comprises a serial-connected P-type transistorand N-type transistor, and wherein a first source/drain of the P-typetransistor is coupled to the power, a second source/drain of the P-typetransistor is coupled to a first source/drain of the N-type transistor,a second source/drain of the N-type transistor is coupled to the halfpower plus the predetermined voltage (ΔV), and a gate of the N-typetransistor and a gate of the P-type transistor are controlled by therail-to-rail differential amplifier.
 7. The half-power buffer amplifierof claim 6, wherein the second-half buffer stage comprises aserial-connected P-type transistor and N-type transistor, and wherein afirst source/drain of the P-type transistor is coupled to the powerminus the predetermined voltage (ΔV), a second source/drain of theP-type transistor is coupled to a first source/drain of the N-typetransistor, a second source/drain of the N-type transistor is coupled tothe ground, and a gate of the N-type transistor and a gate of the P-typetransistor are controlled by the rail-to-rail differential amplifier. 8.The half-power buffer amplifier of claim 7, wherein the rail-to-raildifferential amplifier comprises: a pair of complementary differentialamplifiers having a non-inverting input node for receiving an input andan inverting input node for receiving the output of the first-halfbuffer stage or the output of the second-half buffer stage; and anamplifying stage for amplifying voltage difference between thenon-inverting input node and the inverting input node.
 9. The half-powerbuffer amplifier of claim 8, wherein the amplifying stage comprises: afirst floating current source, associated with the first-half bufferstage, for controlling the gates of the P-type transistor and the N-typetransistor in the first-half buffer stage; and a second floating currentsource, associated with the second-half buffer stage, for controllingthe gates of the P-type transistor and the N-type transistor in thesecond-half buffer stage.
 10. The half-power buffer amplifier of claim9, wherein each of the first floating current source and the secondfloating current source comprises a parallel-connected N-type transistorand P-type transistor with their sources/drains respectively connectedat a first node and a second node, wherein the first node and the secondnode are respectively coupled to the gates of the P-type transistor andthe N-type transistor of the associated first-half or second-half bufferstage.
 11. A half-power buffer amplifier having multiple channels, andcomprising: a rail-to-rail differential amplifier for each channel; abuffer stage associated with the rail-to-rail differential amplifier ofthe same channel, said buffer stage including a first-half buffer stageand a second-half buffer stage, wherein an output of the first-halfbuffer stage is controllably fed back to the rail-to-rail differentialamplifier of the same channel, and an output of the second-half bufferstage is controllably fed back to the rail-to-rail differentialamplifier of the same channel; and a switch network associated with therail-to-rail differential amplifier of the same channel, said switchnetwork including: a first switch connected between the output of thefirst-half buffer stage and an output of the same channel, and a secondswitch connected between the output of the second-half buffer stage andthe output of the same channel, wherein the switch network is controlledin a manner such that a same pixel, with respect to different frames, ofa display panel is driven by the rail-to-rail differential amplifier ofthe same channel.
 12. The half-power buffer amplifier of claim 11,wherein the rail-to-rail differential amplifier comprises an operationalamplifier.
 13. The half-power buffer amplifier of claim 12, wherein theoperational amplifier is powered by a full-range power supply, and inputvoltage level of the operational amplifier is within the full range ofthe power supply.
 14. The half-power buffer amplifier of claim 12,wherein the output of the first-half buffer stage or the output of thesecond-half buffer stage is controllably coupled to an inverting inputnode of the operational amplifier.
 15. The half-power buffer amplifierof claim 11, wherein the first-half buffer stage is coupled to power atone end and coupled to half power plus a predetermined voltage (ΔV) atanother end, and the second-half buffer stage is coupled to the halfpower minus the predetermined voltage (ΔV) at one end and coupled toground at another end.
 16. A half-power buffer amplifier of multiplechannels, comprising: a rail-to-rail differential amplifier for eachchannel; a buffer stage associated with the two rail-to-raildifferential amplifiers of adjacent channels, said buffer stageincluding a first-half buffer stage and a second-half buffer stage,wherein an output of the first-half buffer stage is controllably fedback to one of the two rail-to-rail differential amplifiers of theadjacent channels, and an output of the second-half buffer stage iscontrollably fed back to one of the two rail-to-rail differentialamplifiers of the adjacent channels; and a switch network associatedwith the two rail-to-rail differential amplifiers of the adjacentchannels, said switch network including: a first switch connectedbetween the output of the first-half buffer stage and an output of thefirst of the adjacent channels, a second switch connected between theoutput of the first-half buffer stage and an output of the second of theadjacent channels, a third switch connected between the output of thesecond-half buffer stage and the output of the first of the adjacentchannels, and a fourth switch connected between the output of thesecond-half buffer stage and the output of the second of the adjacentchannels, wherein the switch network is controlled in a manner such thata same pixel, with respect to different frames, of a display panel isdriven by the rail-to-rail differential amplifier of the same channel.17. The half-power buffer amplifier of claim 16, wherein therail-to-rail differential amplifier comprises an operational amplifier.18. The half-power buffer amplifier of claim 17, wherein the operationalamplifier is powered by a full-range power supply, and input voltagelevel of the operational amplifier is within the full range of the powersupply.
 19. The half-power buffer amplifier of claim 17, wherein theoutput of the first-half buffer stage or the output of the second-halfbuffer stage is controllably coupled to an inverting input node of oneof the two operational amplifiers of the adjacent channels.
 20. Thehalf-power buffer amplifier of claim 16, wherein the first-half bufferstage is coupled to power at one end and coupled to half power plus apredetermined voltage (ΔV) at another end, and the second-half bufferstage is coupled to the half power minus the predetermined voltage (ΔV)at one end and coupled to ground at another end.